Following impressive iPhone 4S sales, it’s now only possible to purchase the new handset from U.S. Apple Stores if the smart phone has been reserved online. Apple notes on its “How to Buy” webpage for the iPhone 4S that the handset “is available in store by reservation only.”

iPhone 4S is available in store by reservation only. Reserve yours after 9:00 p.m. tonight for pickup tomorrow. When you come in, you’ll choose a carrier and plan, and we’ll get your iPhone up and running before you leave the store.

According to MacRumors, who contacted an Apple representative, some retail stores are continuing to sell iPhone 4S handset to customers who have not made reservations online. This suggests that reserved iPhone 4S handsets are placed to one side, and that unreserved handsets are then sold to walk-in customers.

However, the representative also noted that iPhone 4S supplies are “evaporating,” suggesting that in the near future, reservations for store-bought iPhone 4S handsets may indeed be essential.